This invention relates to a manufacturing method for a laminated sheet. More particularly, it relates to a manufacturing method for a laminated sheet which has rigidity, heat insulating properties, and shielding properties and can be subjected to deep drawing to form materials suitable for a wide variety of uses including the ceiling, door trim, rear shell, seat back, trunk lid, trunk periphery, floor, and other interior parts of automobiles, the ceilings and walls of buildings, and decorative containers.
In the past, the interior finish of automobiles was made from materials which could withstand temperature in excess of 100.degree. C. were used, such as resin felt whose fibers were packed with phenol-aldehyde condensation resin, expanded synthetic resins, polypropylene complexes, and polypropylene corrugated board. Of these materials, resing felt has excellent stiffness, heat distortion resistance, dimensional stability, and shielding properties, but it is inferior with respect to formability, impact resistance, and lightness. Polypropylene corrugated board has excellent stiffness and lightness, but is is difficult to subject to deep drawing, and being corrugated, it inherently suffers from its strength being directional. Expanded synthetic resins such as polystyrene have superior lightness, but they have poor formability and bending resistance. Furthermore, after forming the surface of an expanded synthetic resin does not have a pleasant feel. Polypropylene complexes have excellent stiffness, but are lacking with respect to formability, impact resistance, and dimensional stability. Thus, there has yet to be found a material which can satisfy all the requirements (stiffness and a suitable flexibility, lightness, dimensional stability, heat distortion resistance, and formability) for interior finish.
A method for manufacturing a nonwoven fabric having excellent elasticity is known in which a fiber made of polyethylene, polypropylene, a low-melting (140.degree. C.) polyester, or the like is used as a fiber binder, a fibrous mat made of such a fiber binder and a synthetic fiber having a higher melting point than the fiber binder is subjected to needling, and the fibers of the upper and lower layers of the web are temporarily entangled with each other, after which the material is heated, and the fiber binder is melted and joined with the other synthetic fiber. This nonwoven fabric has good lightness and flexibility, but it has poor formability and stiffness. Therefore, while it is useful as interior finish for flat surfaces, it can not be employed for locations having a complicated shape.
Another conventional material which can be employed for the interior finish of automobiles is obtained by coating or impregnating a needle punched fabric with an aqueous emulsion of a thermoplastic resin with a softening point of 100.degree.-130.degree. C., after which drying by heating is performed to remove moisture and obtain a formable nonwoven fabric. The fabric is then heated and press molded to obtain an interior finish material. This material has the advantage that it can be applied to a surface having a complex shape. The fibers of this nonwoven fabric are secured by the entwinement of the fibers due to needle punching and by the adhesion of the emulsion resin to the fibers.
However, the nonwoven fabric which is coated or impreganted with the emulsion is bulky with an apparent density of 0.08-0.13 grams per cubic centimeter. Therefore, there is the disadvantage that the filler effect of the emulsion resin is poor. As the filler effect is poor, this material has poor shielding properties.
In Japanese Laid-Open Patent Application No. 58-87353, the present inventor disclosed a manufacturing method for a formable nonwoven fabric which has improved dimensional stability and stiffness, which are drawbacks of the previously-described nonwoven fabric, with no decrease in lightness, heat distortion resistance, and air permeability. In this method, a fibrous mat is employed which comprises 15-50 weight % of a thermoplastic resin fiber binder and 85-50 weight % of a synthetic or natural fiber which has a melting point which is at least 40.degree. C. higher than that of the thermoplastic resin. After the fibrous mat is subjected to needling, it is heated to a temperature at which the thermoplastic resin fiber binder melts but the synthetic or natural fiber does not. While maintaining the thermoplastic resin fiber binder in a molten state, the fibrous mat is compress and the apparent density is adjusted to 0.15-0.50 grams per cubic centimeter. The compressed fibrous mat is then coated or impregnated with an aqueous emulsion of a thermoplastic resin whose formable temperature range is 80.degree.-180.degree. C. in an amount such that the resin solid content of the emulsion is 15-300 weight % with respect to the weight of the fibers in the fibrous mat. The mat is then dryed by heating to 60.degree.-250.degree. C. to remove moisture, thereby obtaining an nonwoven fabric.
In this method, the stiffness of the nonwoven fabric is increased by the combined use of a thermoplastic resin fiber binder and a resin emulsion. Also, the dimensional stability is increased by the bonding of the fibers to one another. Furthermore, by compressing the mat and driving out a portion of the air inside it prior to coating or impregnation of the emulsion, the packing rate of the emulsion resin inside the mat is increased, thereby increasing the stiffness of the nonwoven fabric.
However, while this formable nonwoven fabric has excellent formability, dimensional stability, and air permeability (sound absorbing properties) as well as increased stiffness, on account of its having air permeability, it is inferior with respect to its ability to shield water. Furthermore, depending on the use, a higher stiffness and resistance to water permeation are required.
As a material having superior resistance to water permeation and stiffness, the present inventor has proposed a carpet in which a foamed sheet is bonded to the back surface of a nonwoven fabric which was subjected to resin backing (Japanese Utility Model Applications No. 57-170656 and 57-195731).
However, in this method, a backing layer forming step and a bonding step for the foamed sheet are necessary. Furthermore, in order to prevent breakage due to the bending of the foamed sheet, it is necessary to laminate a non-foamed sheet to the foamed sheet. In addition, as the outer layer is a resin layer, the appearance of the carpet is not satisfactory.
In Japanese Laid-Open Patent Application No. 60-59176, the present inventor disclosed a manufacturing method for a floor carpet having formability and hot adhesion properties. In that method, a first backing material containing (A) a resing aqueous emulsion and (B) expandable polystyrene particles is coated on the back side of a web material. Components (A) and (B) are:
(A) a resin aqueous emulsion whose main component is an aqueous dispersion of a resin with a melting point of at least 80.degree. C. (100 parts by weight in solids), and
(B) expandable polystyrene particles with a particle diameter of at most 1.5 mm (10-100 parts by weight).
After coating, a second backing material is coated on the surface of the first backing material. The second backing material is an aqueous emulsion of a resin having a melting point which is 50.degree.-125.degree. C. and which is lower than the melting point of the resin particles of aqueous emulsion (A). Next, the emulsion is dried by heating and a backing layer is formed. At the same time, the expandable polystyrene particles are expanded.
In this method, the amount of expandable polystyrene particles which are used is only a small amount, i.e., 10-100 parts by weight per 100 parts by weight of the resin in the aqueous emulsion, the foam layer is not a smooth, continuous layer, and spheres of the foam particles protrude into the back side. Therefore, the thickness of the floor carpet is not uniform, and there is the drawback that in order to uniformly bond it to a base of a material such as metal or wood, it is necessary to use a large amount of adhesive in order to fill in the valleys between the protruding foam particles and the web. Furthermore, the appearance of the back side is not satisfactory.
In addition, when it is used as a floor material for an automobile, it is too stiff to provide the elasticity which is required when it is walked upon, and when a laminated sheet employing this material is bent, cracks form in the foam layer.
Furthermore, when this material is used as a floor material for an automobile, there are problems with respect to its ability to fit irregularities in the interior surface of the automobile, which will hereinunder be referred to as its fitting ability.
In order to decrease the stiffness and increase the elasticity of a laminated sheet so as to increase its fitting ability, a resin aqueous emulsion having a low glass transition point can be used. However, in this case, the heat resistance of the laminated sheet decreases. Considering that heat from the engine is transmitted to the passenger compartment of the vehicle and that the inside of the vehicle may reach 60.degree. C. in summer, it is not desirable to lower the glass transition point of the emulsion resin.
The present inventor found that when a crosslinking-type resin aqueous emulsion is used together with a plasticizer, a resin film having a three-dimensional network structure is formed, and a laminated sheet is obtained which has improved fitting ability and is more suitable for being walked upon with only a small decrease in heat resistance.
Furthermore, upon performing various investigations on crosslinking-type resin aqueous emulsions, it was found that when a resin aqueous emulsion of a resin which crosslinks at normal temperatures is used (see Japanese Laid-Open Patent Application Nos. 57-3850, 57-3857, and 58-96643), due to the presence of a plasticizer, the decrease in heat resistance is large, just as when a non-crosslinked resin aqueous emulsion is used. However, when a heat crosslinkable (thermosetting-type) resin aqueous emulsion is used (see Japanese Laid-Open Patent Applications Nos. 58-67762, 58-69253, 58-132051, 59-74166, 60-35059, and 60-212469), there is only a small decrease in heat resistance.